Ring network and data transmitter

ABSTRACT

It is an object to provide a ring-shaped network which can speedily perform initialization even when a plurality of data transmission apparatuses, each performing multi-valued transmission while assigning one or more bits of data as one data symbol to a signal level, are connected thereto, and a data transmission apparatus.  
     The data transmission apparatus outputs received data to a device connected thereto while performing establishment of clock synchronization on the basis of the received data, and when synchronization is established, it resends the received data. On the other hand, when synchronization of the connected device is established, it sends data supplied from the connected device. Further, in the ring-shaped network, a master data transmission apparatus outputs data at turn-on of power or immediately after reset, and a next-stage slave data transmission apparatus receives the data to establish synchronization, and resends the received data when synchronization is established. Thus, synchronization of all the data transmission apparatuses is established while making a round of this data, and then synchronization of devices connected to the respective data transmission apparatuses is established.

TECHNICAL FIELD

[0001] The present invention relates to a ring-shaped network and a data transmission apparatus and, more particularly, to a ring-shaped network in which a data transmission apparatus receives data to establish synchronization, and outputs the data to a next-stage data transmission apparatus when synchronization is established, thereby establishing synchronization while making a round of the data.

BACKGROUND ART

[0002] Conventionally, in order to share information with plural apparatuses, a plurality of data transmission apparatuses each having a transmission unit for transmitting data and a reception unit for receiving data are provided, and the respective data transmission apparatuses are connected in a ring shape to constitute a network.

[0003]FIG. 11 is a block diagram illustrating the construction of a conventional ring-shaped network.

[0004] With reference to FIG. 11, the conventional ring-shaped network is provided with a plurality of data transmission apparatuses, and each data transmission apparatus is connected to a next-stage data transmission apparatus through a transmission path, and a final-stage data transmission apparatus is connected to an initial-stage data transmission apparatus.

[0005] A data transmission apparatus 1010 is provided with a data transmission unit 1011 for transmitting data; an E/O 1014 for converting an electric signal into an optical signal; an optical fiber cable 1016 for transmitting the optical signal; an O/E 1015 for converting an optical signal into an electric signal; a data reception unit 1012 for receiving data; and a clock reproduction unit 1013 for establishing clock synchronization on the basis of the received data. Likewise, a data transmission apparatus 1020 is provided with a data transmission unit 1021, an E/O 1024, an optical fiber cable 1026, an O/E 1025, a data reception unit 1022, and a clock reproduction unit 1023. Further, a data transmission apparatus 1030 is provided with a data transmission unit 1031, an E/O 1034, an optical fiber cable 1036, an O/E 1035, a data reception unit 1032, and a clock reproduction unit 1033.

[0006] The initial-stage data transmission apparatus 1010 is connected to the next-stage data transmission apparatus 1020 through the transmission path 1016, the data transmission apparatus 1020 is connected to the data transmission apparatus 1030 through the transmission path 1026, and the final-stage data transmission apparatus 1030 is connected to the initial-stage data transmission apparatus 1010 through the transmission path 1036, thereby constituting a ring-shaped network.

[0007] Further, the data transmission apparatus 1010 is a master which transmits data with a clock of its own, and the data transmission apparatuses 1020 and 1030 are slaves which transmit data after clock synchronization is established on the basis of the received data.

[0008] Next, the operation of initializing the ring-shaped network so constructed will be described.

[0009] Initially, the data transmission apparatus 1010 that functions as a master transmits data to the next-stage data transmission apparatus 1020 with a clock of its own. In the data transmission apparatus 1020, the clock reproduction unit 1023 performs clock reproduction from the received data to establish synchronization, and thereafter, outputs the data to the next-stage data transmission apparatus 1030. In repetition of this operation, when clock synchronization is established by the clock reproduction unit 1013 of the master data transmission apparatus 1010, the ring-network is initialized. When initialization of the ring-shaped network is completed, the respective data transmission apparatuses perform data communication through the network.

[0010] There are cases where devices are connected to the respective data transmission apparatuses although they are not shown in the figure. These connected devices perform various kinds of processing on the basis of the data received by the data reception unit 1012, 1022, and 1032, and output the results to the data transmission apparatuses 1011, 1021, and 1031, respectively.

[0011] In order to initialize the ring-shaped network in which the data transmission apparatuses having the connected devices are connected, initially, synchronization of the data transmission apparatus 1020 is established, and synchronization of the device connected to the apparatus 1020 is established with the synchronization clock. Thereafter, synchronization of the data transmission apparatus 1010 is established, and synchronization of the device connected to the apparatus 101 is established with the synchronization clock.

[0012] However, in the conventional ring-shaped network, after clock synchronization of the previous-stage data transmission apparatus is established, the previous-stage apparatus outputs a signal to the next-stage data transmission apparatus, and the next-stage data transmission apparatus receives the signal to establish clock synchronization. Therefore, initialization takes much time.

[0013] Further, when devices are connected to the data transmission apparatuses, synchronization must be established in the respective connected devices, and therefore, initialization takes more time.

[0014] Moreover, although FIG. 11 shows an example of a ring-shaped network in which three-stages of data transmission apparatuses are connected, more data transmission apparatuses are connected in an actual ring-shaped network. Therefore, the time required for initialization increases according to the number of data transmission apparatuses connected.

[0015] The present invention is made to solve the above-described problems and has for its object to provide a ring-shaped network and a data transmission apparatus, which can perform initialization at high speed even when performing multi-valued transmission.

DISCLOSURE OF THE INVENTION

[0016] In order to solve the above-described problems, according to the present invention (Claim 1), there is provided a ring-shaped network in which plural data transmission apparatuses, each performing data transmission while assigning one or more bits of data as one data symbol to a signal level, are connected via a transmission path, wherein one of the data transmission apparatuses is a master data transmission apparatus while the other data transmission apparatuses are slave data transmission apparatuses; each of the data transmission apparatuses is connected with a device which is supplied with data received by the data transmission apparatus, and outputs a result of processing to the data transmission apparatus; at turn-on of power or immediately after reset, the master data transmission apparatus outputs the data supplied from the connected device, to the transmission path; each of the slave data transmission apparatuses receives, from the transmission path, the data outputted from the data transmission apparatus connected in the previous stage, establishes clock synchronization, outputs the received data to the connected device, and resends the received data to the transmission path; and the slave data transmission apparatus outputs the data supplied from the connected device, to the transmission path, after synchronization of the connected device is established.

[0017] Further, according to the present invention (Claim 2), in the ring-shaped network defined in Claim 1, the slave data transmission apparatus does not output the data to the transmission path until establishment of clock synchronization is completed.

[0018] Further, according to the present invention (Claim 3), there is provided a data transmission apparatus which is connected to a ring-shaped network through a transmission path, and performs data transmission while assigning one or more bits of data as one data symbol to a signal level; and the data transmission apparatuses is connected with a device which is supplied with data received by the data transmission apparatus, and outputs a result of processing to the data transmission apparatus; and the data transmission apparatus includes a synchronization detector for establishing synchronization on the basis of the data inputted through the transmission path, and performing reproduction of a data clock, a connected device synchronization detector for detecting that synchronization of the device is established, and a selector for switching data to be transmitted to the transmission path between the data received through the transmission path and the data supplied from the device.

[0019] Further, according to the present invention (Claim 4), in the data transmission apparatus defined in Claim 3, the connected device synchronization detector receives the data from the connected device, and reproduces a device clock that is synchronized with the data.

[0020] Further, according to the present invention (Claim 5), in the data transmission apparatus defined in Claim 3 or Claim 4, the connected device synchronization detector receives the data supplied from the connected device, and detects a data frame including the data, thereby to detect that synchronization of the connected device is established.

[0021] Further, according to the present invention (Claim 6), in the data transmission apparatus defined in Claim 3, the connected device synchronization detector counts the clock that is inputted in synchronization with the data supplied from the connected device, and the data clock that is reproduced by the synchronization detector on the basis of the data received through the transmission path, for a predetermined period of time, thereby to detect that synchronization of the connected device is established.

[0022] Further, according to the present invention (Claim 7), in the data transmission apparatus defined in Claim 4, the connected device synchronization detector counts the device clock that is reproduced from the data supplied from the connected device, and the data clock that is reproduced by the synchronization detector on the basis of the data received through the transmission path, for a predetermined period of time, thereby to detect that synchronization of the connected device is established.

[0023] As described above, according to the present invention (Claim 1), there is provided a ring-shaped network in which plural data transmission apparatuses, each performing data transmission while assigning one or more bits of data as one data symbol to a signal level, are connected via a transmission path, wherein one of the data transmission apparatuses is a master data transmission apparatus while the other data transmission apparatuses are slave data transmission apparatuses; each of the data transmission apparatuses is connected with a device which is supplied with data received by the data transmission apparatus, and outputs a result of processing to the data transmission apparatus; at turn-on of power or immediately after reset, the master data transmission apparatus outputs the data supplied from the connected device, to the transmission path; the slave data transmission apparatus receives, from the transmission path, the data outputted from the data transmission apparatus connected in the previous stage, establishes clock synchronization, outputs the received data to the connected device, and resends the received data to the transmission path; and the slave data transmission apparatus outputs the data supplied from the connected device, to the transmission path, after synchronization of the connected device is established. Therefore, synchronization of the respective data transmission apparatuses can be established on the basis of the received data to initialize the ring-shaped network. Further, synchronization of the next-stage data transmission apparatus can be established by resending the data to the apparatus without waiting for completion of establishment of clock synchronization in the connected device, whereby the ring-shaped network can be initialized more speedily.

[0024] Further, according to the present invention (Claim 2), in the ring-shaped network defined in Claim 1, the slave data transmission apparatus does not output the data to the transmission path until establishment of clock synchronization is completed. Therefore, the data can be transmitted after clock synchronization of the slave data transmission apparatus is established.

[0025] Further, according to the present invention (Claim 3), there is provided a data transmission apparatus which is connected to a ring-shaped network via a transmission path, and performs data transmission while assigning one or more bits of data as one data symbol to a signal level; and the data transmission apparatuses is connected with a device which is supplied with data received by the data transmission apparatus, and outputs a result of processing to the data transmission apparatus; and the data transmission apparatus includes a synchronization detector for establishing synchronization on the basis of the data inputted through the transmission path, and performing reproduction of a data clock, a connected device synchronization detector for detecting that synchronization of the device is established, and a selector for switching data to be transmitted to the transmission path between the data received through the transmission path and the data from the device. Therefore, the data transmission apparatus resends the received data until synchronization of the connected device is established, and it can output the data supplied from the connected device when synchronization of the connected device has been established.

[0026] Further, according to the present invention (Claim 4), in the data transmission apparatus defined in Claim 3, the connected device synchronization detector receives the data supplied from the connected device, and reproduces a device clock that is synchronized with the data. Therefore, the data transmission apparatus can perform processing even when no clock is supplied from the connected device.

[0027] Further, according to the present invention (Claim 5), in the data transmission apparatus defined in Claim 3 or Claim 4, the connected device synchronization detector receives the data supplied from the connected device, and detects a data frame including the data, thereby to detect that synchronization of the connected device is established. Therefore, the data transmission apparatus can detect synchronization of the connected device.

[0028] Further, according to the present invention (Claim 6), in the data transmission apparatus defined in Claim 3, the connected device synchronization detector counts the clock that is inputted in synchronization with the data supplied from the connected device, and the data clock that is reproduced by the synchronization detector on the basis of the data received through the transmission path, for a predetermined period of time, thereby to detect that synchronization of the connected device is established. Therefore, the data transmission apparatus can detect establishment of synchronization of the connected device even when the inputted data is not stored in a frame.

[0029] Further, according to the present invention (Claim 7), in the data transmission apparatus defined in Claim 4, the connected device synchronization detector counts the device clock that is reproduced from the data supplied from the connected device, and the data clock that is reproduced by the synchronization detector on the basis of the data received through the transmission path, for a predetermined period of time, thereby to detect that synchronization of the connected device is established. Therefore, the data transmission apparatus can detect establishment of synchronization of the connected device even when the inputted data is not stored in a frame. Further, it can perform processing even when no clock is supplied from the connected device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a block diagram illustrating the construction of a data transmission apparatus according to a first embodiment of the present invention.

[0031]FIG. 2 is a block diagram illustrating the construction of a ring-shaped network to which the data transmission apparatus according to the first embodiment of the present invention is connected.

[0032]FIG. 3 is a diagram illustrating an example of a digital data string to be transmitted by the data transmission apparatus according to the first embodiment of the present invention.

[0033]FIG. 4 is a diagram for explaining conversion of data symbols into signal levels (mapping values) by the data transmission apparatus according to the first embodiment of the present invention.

[0034]FIG. 5 is a diagram illustrating an example of a waveform which is outputted onto a transmission path by the data transmission apparatus according to the first embodiment of the present invention.

[0035]FIG. 6 is a flowchart for explaining the operation of the ring-shaped network according to the first embodiment of the present invention for establishing clock synchronization.

[0036]FIG. 7 is a block diagram illustrating the construction of a connected device synchronization detector according to the first embodiment of the present invention.

[0037]FIG. 8 is a block diagram illustrating another example of the construction of the connected device synchronization detector according to the first embodiment of the present invention.

[0038]FIG. 9 is a block diagram illustrating another example of the construction of the connected device synchronization detector according to the first embodiment of the present invention.

[0039]FIG. 10 is a block diagram illustrating another example of the construction of the connected device synchronization detector according to the first embodiment of the present invention.

[0040]FIG. 11 is a block diagram illustrating the construction of the conventional ring-shaped network.

BEST MODE TO EXECUTE THE INVENTION

[0041] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The embodiment described hereinafter is merely an example, and the present invention is not necessarily restricted thereto.

[0042] (Embodiment 1)

[0043] A ring-shaped network as defined in Claim 1 or 2, and a data transmission apparatus as defined in any of Claims 3 to 7 will be described as a first embodiment of the invention, with reference to the drawings.

[0044]FIG. 1 is a block diagram illustrating the construction of a data transmission apparatus according to the first embodiment.

[0045] With reference to FIG. 1, a data transmission apparatus 100 according to the first embodiment comprises a transmission unit 110 for outputting data, a reception unit 120 for receiving the data outputted from the transmission unit 110, and a transmission path 130 for transmitting the data outputted from the transmission unit 110. A device 200 is connected to this data-transmission apparatus 100, and the connected device 200 performs processing on the basis of the data outputted from the reception unit 120, and outputs the result to the transmission unit 110.

[0046] The transmission unit 110 is provided with a connected device synchronization detector 111 for detecting establishment of synchronization in the connected device 200; a selector 112 for selecting data to be transmitted; a data mapping unit 113 for mapping a digital signal to be transmitted to any of eight signal levels at every symbol timing; a digital filter 114 which allows a band of frequency components corresponding to half of the symbol rate to pass; a D/A converter 115 for converting digital data into an analog signal; a low-pass filter 116 for removing noises outside the signal band of the transmitted signal; and a driver 117 for amplifying the signal intensity, and outputting the signal to the transmission path 130.

[0047] On the other hand, the reception unit 120 is provided with a receiver 121 for receiving a signal; a low-pass filter 122 for removing noises outside the signal band; an A/D converter for converting the signal into digital data; a digital filter 124 which allows a band of frequency components corresponding to half of the symbol rate to pass; an evaluation unit 125 for decoding data symbols; and a synchronization detection unit 126 for establishing synchronization.

[0048] Further, a coaxial cable or a twisted-pair line is employed as the transmission path 130. When the transmission path 130 is a twisted-pair line, the driver 117 must be a differential output type driver.

[0049]FIG. 2 is a block diagram illustrating the construction of a ring-shaped network according to the first embodiment.

[0050] With reference to FIG. 2, the ring-shaped network according to the first embodiment comprises n stages of data transmission units, each comprising a data transmission apparatus which performs transmission and reception of data, and a connected device which performs processing on the basis of data outputted from the data transmission apparatus to output the result to the data transmission apparatus, and the n stages of data transmission units are connected in a ring shape such that an output from a previous-stage data transmission unit is input to a next-stage data transmission unit, and an output from a final-stage data transmission unit is input to an initial-stage data transmission unit.

[0051] All of the data transmission apparatuses are identical in construction to the data transmission apparatus 100. That is, the first data transmission apparatus 100 a is provided with a transmission unit 110 a, a reception unit 120 a, and a transmission path 130 a although these elements are not shown in FIG. 2, and further, it is connected with a device 200 a which performs processing on the basis of the data outputted from the reception unit 120 a to output the result to the transmission unit 110 a. The transmission unit 110 a is provided with a connected device synchronization detector 111 a, a selector 112 a, a data mapping unit 113 a, a digital filter 114 a, a D/A converter 115 a, a low-pass filter 116 a, and a driver 117 a. The reception unit 120 a is provided with a receiver 121 a, a low-pass filter 122 a, an A/D converter 123 a, a digital filter 124 a, an evaluation unit 125 a, and a synchronization detector 126 a.

[0052] Likewise, the second data transmission apparatus 100 b is provided with a transmission unit 110 b, a reception unit 120 b, and a transmission path 130 b, and further, it is connected with a device 200 b which performs processing on the basis of the data outputted from the reception unit 120 b to output the result to the transmission unit 110 b. The transmission unit 110 b is provided with a connected device synchronization detector 111 b, a selector 112 b, a data mapping unit 113 b, a digital filter 114 a, a D/A converter 115 b, a low-pass filter 116 b, and a driver 117 b. The reception unit 120 b is provided with a receiver 121 b, a low-pass filter 122 b, an A/D converter 123 b, a digital filter 124 a, an evaluation unit 125 b, and a synchronization detector 126 b.

[0053] Furthermore, the final-stage data transmission apparatus 100 n is provided with a transmission unit 10 n, a reception unit 120 n, and a transmission path 130 n, and further, it is connected with a device 200 n which performs processing on the basis of the data outputted from the reception unit 120 n to output the result to the transmission unit 110 n. The transmission unit 110 n is provided with a connected device synchronization detector 111 n, a selector 112 n, a data mapping unit 113 n, a digital filter 114 n, a D/A converter 115 n, a low-pass filter 116 n, and a driver 117 n. The reception unit 120 n is provided with a receiver 121 n, a low-pass filter 122 n, an A/D converter 123 n, a digital filter 124 n, an evaluation unit 125 n, and a synchronization detector 126 n.

[0054] The transmission unit 110 a of the first data transmission apparatus 100 a is connected to the reception unit 120 b of the second data transmission apparatus 100 b through the transmission path 130 a, the transmission unit 10 b of the second data transmission apparatus 100 b is connected to the reception unit 120 c of the third data transmission apparatus 100 c through the transmission path 130 b, and the transmission unit 110 n of the final-stage (n-th) data transmission apparatus 100 n is connected to the reception unit 120 a of the initial-stage (first) data transmission apparatus 100 a through the transmission path 130 n, thereby constituting the ring-shaped network.

[0055] The first data transmission apparatus 10 a functions as a master, and it transmits data with a clock of its own at turn-on of power or immediately after reset. On the other hand, the data transmission apparatuses 100 b to 100 n function as slaves, and these apparatuses establish clock synchronization on the basis of the received data at turn-on of power or immediately after reset and, thereafter, output the data.

[0056] Next, the operation of the data transmission apparatus 100 will be described.

[0057]FIG. 3 is a diagram illustrating an example of a digital data string to be transmitted. The data mapping unit 113 combines every three bits of data in the digital data string to make data symbols, and maps each data symbol to a signal level according to a conversion table shown in FIG. 4. FIG. 4 is a conversion table illustrating conversion from data symbols to signal levels (mapping values), wherein each data symbol comprising three bits of data is mapped to any of eight signal levels. In the example shown in FIG. 3, a symbol “100” is mapped to a signal level “+1” at a certain symbol timing. Likewise, symbols “101”, “110”, “011”, and “011” are mapped to signal levels “+3”, “+5”, “−1”, and “−1” at the subsequent symbol timings, respectively.

[0058] Hereinafter, the data transmission apparatus 100 will be described as one performing such 8-value transmission.

[0059] Initially, ordinary data transmission will be described. During ordinary data transmission, the selector 112 selects transmission data supplied from the connected device 200, and inputs the data to the data mapping unit 113. Then, the data mapping unit 113 divides the transmission data into groups of three bits, and maps each 3-bit data to one of eight signal levels on the basis of the conversion table shown in FIG. 4. The digital filter 114 is a low-pass filter that allows a band of frequency components corresponding to half of the symbol rate to pass, and actually it is constituted so as to have appropriate roll-off characteristics in combination with the digital filter 124 of the reception unit 120. The signal that has passed through the digital filter 114 is converted into an analog signal by the D/A converter 115, and the analog signal is passed through the low-pass filter 116, whereby a signal within a band that is slightly larger than half of the symbol rate is obtained, and a signal having no interference between adjacent codes is obtained at readout timing. Thereby, data transmission in the finite band can be carried out. Then, the driver 117 amplifies the signal intensity of this signal, and outputs the signal to the transmission path 130.

[0060] An example of a waveform thus outputted onto the transmission path is shown in FIG. 5. In the example of FIG. 5, the signal levels “+1”, “+3”, “+5”, “−1”, and “−1” which are mapped by the data mapping unit 113 are outputted in a waveform.

[0061] The signal so transmitted is received by the reception unit 120.

[0062] In the reception unit 120, the receiver 121 receives the signal, and the low-pass filter 122 removes noises outside the signal band. Then, the A/D converter 123 converts the signal into digital data, and the digital data is passed through the digital filter 124. The digital filter 124 has roll-off characteristics in combination with the digital filter 114, and it converts the digital data into a signal that can be received without being affected by interference between adjacent codes, at appropriate timing. Thereafter, the evaluation unit 125 decodes the signal levels to data symbols, and inputs the received data to the connected device 200 and to the selector 112 of the transmission unit 110. The synchronization detector 126 reproduces a clock from the received data, and establishes synchronization when decoding the data symbols. Ordinary data transmission is carried out as described above.

[0063] Next, the method for establishing clock synchronization of the ring-shaped network so constructed will be described with reference to FIG. 6.

[0064]FIG. 6 is a flowchart for explaining the operation of the ring-shaped network for establishing clock synchronization. Here, a ring-shaped network comprising three stages of data transmission apparatuses 100 a to 100 c will be described.

[0065] At turn-on of power or immediately after reset, the transmission unit 110 a of the first data transmission apparatus 100 a outputs transmission data. That is, the selector 112 a inputs the transmission data to the data mapping unit 113 a. Then, the transmission data mapped to the signal levels are converted into an analog signal in like manner as the ordinary data transmission, and the analog signal is output to the transmission path 130 a (step S201).

[0066] The transmission data is received by the reception unit 120 b of the second data transmission apparatus 100 b. On receipt of the transmission data, the synchronization detector 126 b performs clock reproduction to establish synchronization. Further, the signal levels of the received data are converted into a digital signal in like manner as the ordinary data transmission, and the digital signal is output to the connected device 200 b and to the selector 112 b of the transmission unit 110 b. When the synchronization detector 126 b has established synchronization, it outputs a synchronization detection signal to the selector 112 b of the transmission unit 110 b (step S202).

[0067] When synchronization is established by the synchronization detector 126 b, the reception unit 120 b goes into the state where it can perform data reception (step S203).

[0068] On the other hand, when the synchronization detection signal is input to the selector 112 b, the transmission unit 110 b resends the data received by the reception unit 120 b. That is, the selector 112 b inputs the received data that is supplied from the evaluation unit 125 b, to the data mapping unit 113 b. Then, the mapped data is resent to the transmission path 130 b in like manner as the ordinary data transmission (step S204).

[0069] Since, at this time, the connected device 200 b establishes synchronization on the basis of the reception data supplied from the reception unit 120 b, the transmission unit 110 b detects establishment of synchronization in the connected device 200 b by the connected device synchronization detector 111 b.

[0070] An example of the construction of the connected device synchronization detector 111 b is shown in FIG. 7, wherein the detector 111 b has an FIFO 801 and a synchronization detector 802. The FIFO 801 is a ring buffer for delaying the transmission data supplied from the connected device 200 b, and the FIFO 801 reads the data from a position indicated by a pointer to output it to the selector 112 b. The synchronization detector 802 detects a data frame including data on the basis of the transmission data and transmission clock from the connected device 200 b. For example, it periodically detects a frame header included in a data frame, thereby detecting establishment of synchronization in the connected device 200 b. When the synchronization detector 802 has detected establishment of synchronization, it outputs a connected device synchronization detection signal to the selector 112 b. Further, the pointer for indicating the position where data are outputted from the FIFO 801 is placed in a predetermined position, and readout of data is carried out at the reproduction clock that is reproduced by the reception unit 120 b. That is, the connected device 200 b performs processing on the basis of the data outputted from the first data transmission apparatus 100 a in the previous stage, and the result of processing is output to the selector 112 b (step S205).

[0071] When synchronization of the connected device 200 b has been established, the selector 112 b selects the data from the connected device 200 b as transmission data, whereby the transmission unit 110 b goes into the state where it can output the transmission data (step S206).

[0072] The transmission data outputted from the second data transmission apparatus 100 b is received by the reception unit 120 c of the third data transmission apparatus 100 c. When the reception unit 120 c receives the transmitted data, the synchronization detector 126 c performs clock reproduction to establish synchronization. The signal levels of the received data are converted into a digital signal in like manner as the ordinary data transmission, and the digital signal is output to the connected device 200 c and to the selector 112 c of the transmission unit 110 c. When the synchronization detector 126 c has established synchronization, it outputs a synchronization detection signal to the selector 112 c of the transmission unit 110 c (step S207).

[0073] When the synchronization detector 126 c has established synchronization, the reception unit 120 c is in the state where it can perform data reception (step S208).

[0074] On the other hand, when the synchronization detection signal is input to the selector 112 c, the transmission unit 110 c resends the reception data received by the reception unit 120 c. That is, the selector 112 c inputs the reception data supplied from the evaluation unit 125 c to the data mapping unit 113 c. The mapped reception data is resent to the transmission path 130 c in the same manner as the ordinary data transmission (step S209).

[0075] Since, at this time, the connected device 200 c establishes synchronization on the basis of the reception data that is supplied from the reception unit 120 c, the transmission unit 110 c detects establishment of synchronization in the connected device 200 c by the connected device synchronization detector 111 c.

[0076] The connected device synchronization detector 111 c is identical in construction to the connected device synchronization detector 111 b. That is, the FIFO 801 delays the transmission data from the connected device 200 c, and reads the data from the position indicated by the pointer to output it to the selector 112 c. Further, the synchronization detector 802 detects synchronization of the connected device 200 c on the basis of the transmission data and transmission clock from the connected device 200 c. When the synchronization detector 802 detects synchronization, it outputs a connected device synchronization detection signal to the selector 112 c. Further, the pointer of the FIFO 801 is placed in a predetermined position, and readout of data is carried out with the reproduction clock reproduced by the reception unit 120 c. That is, the connected device 200 c performs processing on the basis of the data supplied from the second data transmission apparatus 100 b in the previous stage, and outputs the result to the selector 112 c (step S210).

[0077] When synchronization of the connected device 200 c has been established, the selector 112 c selects the data from the connected device 200 c as transmission data, and the transmission unit 110 c is in the state where it can output the transmission data (step S211).

[0078] The transmission data outputted from the third data transmission apparatus 100 c is received by the reception unit 120 a of the first data transmission apparatus 100 a. When the transmission data is received by the reception unit 120 a, the synchronization detector 126 a performs clock reproduction to establish synchronization. Further, the signal levels of the reception data are converted into a digital signal to be output to the connected device 200 a (step S212).

[0079] When synchronization has been established by the synchronization detector 126 a, the reception unit 120 a is in the state where it can perform data reception (step S213).

[0080] When clock synchronization has been established in the data transmission apparatuses 10 a to 100 c constituting the ring-shaped network and the connected devices 200 a to 200 c as described above, transmission and reception of data through the network are started.

[0081] As described above, the data transmission apparatus 100 according to the first embodiment is provided with the synchronization detector 126 for establishing synchronization on the basis of data inputted through the transmission path 130, and reproducing a data clock; the connected device synchronization detector 111 for detecting that synchronization of the connected device 200 is established; and the selector 112 for selecting either the data received by the reception unit 120 or the data supplied from the connected device 200, and outputting the selected data to the transmission path 130. Therefore, the data transmission apparatus 100 resends the data received by the reception unit 120 until synchronization of the connected device 200 is established, and it can send the data supplied from the connected device 200 after establishment of synchronization of the connected device 200.

[0082] Further, in the ring-shaped network in which a plurality of data transmission apparatuses 100 as described above are connected in a ring shape, one of the data transmission apparatuses 100 serves as a master while the other data transmission apparatuses 100 serve as slaves. At turn-on of power or immediately after reset, the first data transmission apparatus 100 a as a master outputs the data from the connected device 200 a to the transmission path 130 a, and the second data transmission apparatus 100 b as a slave receives the data to establish clock synchronization. Then, the apparatus 100 b outputs the received data to the transmission path 130 b and to the connected device 200 b, thereby establishing synchronization of all the data transmission apparatuses connected in the network as well as synchronization of the connected device 200. Thereafter, data to be transmitted to the transmission path 130 is switched to the data supplied from the connected device 200. Therefore, synchronization of the respective data transmission apparatuses 100 can be established, whereby the ring-shaped network can be initialized. Further, since synchronization of the next-stage data transmission apparatus can be established by resending the data without waiting for completion of establishment of clock synchronization in the connected device 200, the ring-shaped network can be initialized more speedily.

[0083] Further, the second to n-th data transmission apparatuses 100 b to 100 n as slaves do not transmit data to the transmission paths 130 b to 130 n until establishment of clock synchronization is completed, data can be transmitted after establishment of clock synchronization in the second to n-th data transmission apparatuses 100 b to 100 n.

[0084] In this first embodiment, as shown in FIG. 4, each 3-bit data is mapped to any of the eight signal levels “−7”, “−5”, “−3”, “−1”, “+1”, “+3”, “+5”, and “+7” in the data mapping unit 113. However, the present invention is not restricted to such 8-value transmission, and arbitrary multi-valued transmission can be carried out.

[0085] Further, although the connected device synchronization detector 111 shown in FIG. 7 is described as a connected device synchronization detector, this is merely an example, and other connected device synchronization detectors 111-1 to 111-3 may be employed.

[0086]FIG. 8 is a block diagram illustrating another example of the construction of the connected device synchronization detector. For example, in the case where no transmission clock is input to the connected device synchronization detector 111, a connected device synchronization detector 111-1 may be employed, which includes a clock reproduction unit 901 that performs clock reproduction on the basis of the transmission data supplied from the connected device 200 to establish synchronization. The synchronization detector 802 detects a data frame including the data on the basis of the transmission data supplied from the connected device 200 and the device clock supplied from the clock reproduction unit 901, and detects establishment of synchronization of the connected device 200. When synchronization is detected, readout of data is carried out with the reproduction clock that is obtained in the reception unit 120.

[0087] Since clock reproduction is carried out inside the connected device synchronization detector 111 as described above, establishment of synchronization can be detected even when no transmission clock is inputted.

[0088] Further, FIGS. 9 and 10 are block diagrams illustrating other examples of the construction of the connected device synchronization detector. For example, in the case where the transmission data is not stored in a frame, a connected device synchronization detector 111-2 or 111-3 may be employed, which detects synchronization by inputting the reproduction clock obtained by the reception unit 120 to the synchronization detector 802, and detecting matching of the reproduction clock with the transmission clock supplied from the connected device 200 or the clock reproduced from the data supplied from the connected device 200.

[0089] In the connected device synchronization detector 111-2 shown in FIG. 9, the transmission clock from the connected device 200 and the reproduction clock are input to the synchronization detector 802. Then, the synchronization detector 802 counts the transmission clock and the reproduction clock for a predetermined period of time to detect frequency matching, thereby detecting that synchronization of the connected device 200 is established.

[0090] Further, in the connected device synchronization detector 111-3 shown in FIG. 10, the device clock reproduced by the clock reproduction unit 901 and the reproduction clock are input to the synchronization detector 802. Then, the synchronization detector 802 counts the device clock and the reproduction clock for a predetermined period of time to detect frequency matching, thereby detecting that synchronization of the device is established.

[0091] In this way, since each of the connected device synchronization detectors 111-2 and 111-3 detects synchronization by detecting matching of the reproduction clock with the clock reproduced from the data supplied from the connected device 200, even when the transmission data is not stored in a frame, establishment of synchronization can be detected.

Applicability in Industry

[0092] A data transmission apparatus according to the present invention performs establishment of synchronization on receipt of data, and outputs the data to a connected device and, when establishment of synchronization is completed, the apparatus outputs the data to a transmission path. Therefore, in a ring-shaped network constituted by plural data transmission apparatuses, synchronization of all data transmission apparatuses can be established while making a round of data, without waiting for establishment of synchronization in connected devices, whereby initialization of the ring-shaped network can be carried out at high speed. 

1. (Amended) A ring-shaped network in which plural data transmission apparatuses, each performing data transmission while assigning one or more bits of data as one data symbol to a signal level, are connected via a transmission path, wherein one of the data transmission apparatuses is a master data transmission apparatus, while the other data transmission apparatuses are slave data transmission apparatuses; each of the data transmission apparatuses is connected with a device which is supplied with data received by the data transmission apparatus, and outputs a result of processing to the data transmission apparatus; at turn-on of power or immediately after reset, the master data transmission apparatus outputs the data supplied from the connected device, to the transmission path; each of the slave data transmission apparatuses receives, from the transmission path, the data outputted from the data transmission apparatus connected in the previous stage, establishes clock synchronization, outputs the received data to the connected device, and resends the received data to the transmission path; and the slave data transmission apparatus outputs the data supplied from the connected device, to the transmission path, after synchronization of the connected device is established.
 2. A ring-shaped network as defined in claim 1, wherein the slave data transmission apparatus does not output the data to the transmission path until establishment of clock synchronization is completed.
 3. A data transmission apparatus which is connected to a ring-shaped network via a transmission path, and performs data transmission while assigning one or more bits of data as one data symbol to a signal level; said data transmission apparatuses being connected with a device which is supplied with data received by the data transmission apparatus, and outputs a result of processing to the data transmission apparatus; and said data transmission apparatus including: a synchronization detector for establishing synchronization on the basis of the data inputted through the transmission path, and performing reproduction of a data clock; a connected device synchronization detector for detecting that synchronization of the connected device is established; and a selector for switching data to be transmitted to the transmission path between the data received through the transmission path and the data supplied from the connected device.
 4. A data transmission apparatus as defined in claim 3, wherein said connected device synchronization detector receives the data supplied from the connected device, and reproduces a device clock that is synchronized with the data.
 5. A data transmission apparatus as defined in claim 3 or claim 4, wherein said connected device synchronization detector receives the data supplied from the connected device, and detects a data frame including the data, thereby to detect that synchronization of the connected device is established.
 6. A data transmission apparatus as defined in claim 3, wherein said connected device synchronization detector counts the clock that is inputted in synchronization with the data supplied from the connected device, and the data clock that is reproduced by the synchronization detector on the basis of the data received through the transmission path, for a predetermined period of time, thereby to detect that synchronization of the connected device is established.
 7. A data transmission apparatus as defined in claim 4, wherein said connected device synchronization detector counts the device clock that is reproduced from the data supplied from the connected device, and the data clock that is reproduced by the synchronization detector on the basis of the data received through the transmission path, for a predetermined period of time, thereby to detect that synchronization of the connected device is established. 